Asteroid Dust

Asteroid Dust

Perhaps, but Olsson is not looking for just any dust. His is interplanetary dust, zodiacal dust, the dust that sifts slowly down through our atmosphere some 25,000 years after it rubs off an asteroid bumped by a pebble-sized rock out beyond the orbit of Mars. This dust accumulates on Earth today at the rate of 40,000 tons per year—an estimate based on the haul of a U2 airplane cruising the stratosphere. "Since it's so small," Olsson says, meaning a single mote of asteroid dust, "it comes in very slowly, compared, for instance, to a meteorite. It stays high in the stratosphere for quite a while. It never gets tremendous acceleration, so it never burns up."

Ages and ages ago, when the asteroid belt was busier, the rocks larger and colliding more often, the influx could have been much, much higher. High enough, maybe, to have affected Earth's climate, or to have introduced organic molecules or metals that helped or hindered early life. These are the questions that fascinate Olsson, an astronomer turned geoscientist ("I've been called a 'physical-astro-bio-geo-chemist,'" he jokes). "Apart from sunlight," he says, "this is the single most important interaction we have with space. This is the channel between the Solar System and Planet Earth."

But Olsson's search for zodiacal dust also has a more practical aim: Early in the next century, NASA plans to launch a probe called the "Terrestrial Planet Finder" to search for other planets in the universe that may be capable of supporting life. The Planet Finder will use the techniques of infrared astronomy to look for signs of water, a requirement for Earth-like life, and ozone, a biproduct of oxygen that could signal plant life. But whether it sees other worlds or not may depend on our knowledge of dust.

"The problem is," says Olsson, "no one has seen the smallest asteroidal grains on the ground, even though we know they must be there." Looking for dust in sediments, he says, "would be like looking for rock in rock." The dust captured in the stratosphere, on the other hand, "is contaminated with all kinds of things: fuel droplets from the Space Shuttle, cometary dust." Nor would it be cost effective to send out a satellite simply to gather dust. "When I say the Solar System is very dirty," Olsson explains, "I mean that relatively. It's cleaner than any vacuum chamber in a lab. There's only one particle of asteroid dust per cubic kilometer. And these dust particles are so small—micron-sized. They're smaller than the dust you see floating in the air when the sun shines into your living room."

The best place to look, Olsson believes, is in ice. As a Penn State graduate student, Olsson is working with Richard Alley, one of the leaders of an international team studying the ice cores drilled in Greenland and Antarctica. (Olsson's second adviser, James Kasting, is a science adviser for NASA's Terrestrial Planet Finder.) These cores contain ice from as deep as two miles beneath the polar caps. As the snow falls, year to year, it compresses into bands of ice that can be read like tree rings, giving a record of Earth's atmosphere spanning hundreds of thousands of years. Asteroid dust should stand out as a layer of rock in the ice—distinguished from volcanic ash or other earthly types of dust by increased concentrations of helium 3, aluminum 26, and beryllium 10, which are a result of contact with the solar wind. Yet even with Alley's assistance, Olsson has to compete for an ice sample. "You need very good data before you can request a sample," he explains. He needs to know precisely when in geologic time—and thus, where in an ice core—he's most likely to find dust.

To pinpoint those times, Olsson has made a computer model of the dusty Solar System. "I start with a database that gives me the positions of the planets and of a couple of hundred asteroids every ten years for the past 400,000 years," he explains. "Then I pick a time when a lot of asteroids are in the same area—when the probability of a collision is high—and I simulate a collision." After the crash, Olsson follows the paths of 150 to 200 individual particles of dust produced (once he tracked 15,000 particles, just to test his method). Scattered by the force of the collision, the tiny dust particles are pushed and pulled by the gravity fields of the planets and by radiative friction from the sun. "Dust this small will 'feel' sunlight," Olsson explains. "It's like ordinary friction but a bit more complicated. Basically, radiative friction causes the orbit of the dust particle to shrink. The dust will spiral inward toward the sun."

A snapshot of all the particles in motion shows a diffuse cloud of asteroid dust circling the sun. Crisscrossed through it are bands where the dust is clumped together, making the cloud thicker. These bands of higher-density dust correspond to several "families" of asteroids. "The asteroid families formed during large collisions billions of years ago," Olsson says, "and the debris still travels in almost the same orbit as the colliding bodies once did." Displaying the cloud on his computer monitor in 3-D, Olsson (wearing blue-and-red-lensed glasses) can spin it on its axes to more clearly see the thick and thin pattern of bands. Overlaying Earth's orbit, as it has changed over time relative to Jupiter's orbit, he can predict when in geological time Earth might have revolved within a thick band of asteroid dust, and how much of it sifted through the atmosphere to leave a dusty layer on the polar ice.

So far he finds these Dust Ages come in pairs, 10,000 years apart, with a clean period of 100,000 years in between. "I've added everything to my model that I can think of," he says, "and it still gives me the same answers. That the pattern is not perfectly cyclic makes it easier to check. The peaks should be easy to find in the ice record, if they're there. I hope I can check it myself.

"By the way," he adds, "Most of the metallic dust on your bookshelf and under the bed, and in particular in plastic drainpipes, is usually either asteroidal dust or cometary dust. The particles are much larger than the ones I'm looking for. Still it's something to ponder when mopping the floors."

Bertil Olsson is a graduate student in geosciences in the College of Earth and Mineral Sciences, 436 Deike Bldg., University Park, PA 16802; 814-865-3321; olsson @essc.psu.edu. His advisers are Richard Alley, Ph.D., professor of geosciences, 204 Deike Bldg.; 863-1700; ralley@essc.psu. edu; and James Kasting, Ph.D., professor of geosciences and meteorology, 443 Deike Bldg.; 865-3207; kasting@essc.psu.edu. Olsson's research is funded by the National Science Foundation, the David and Lucile Packard Foundation, and NASA.